We get asked a lot about tire size and how it affects HP. Yes you will lose a little HP because of larger tires, but that is not the main reason performance suffers. It has more to do with your RPM band and how changing tires size move where you are at in the rpms at a certain speed.

Check out this video to learn more.

 

Nice explanation. Thanks for sharing.

 

horsepower at the back of the engine does not change... just the SAE horsepower changes, IE that which you feel... the mechanical gear ratio changed along with unsprung weight... as a person talking about tuning learn the difference.

 

He took a common issue/oversight/lack of understanding that many people run into and dont think about or understand, and made a simple video explaining it to help. It wasnt meant to be an engineering course. It is a very common problem that many who go to oversize tires seem to not realize until after the fact.

“as a person talking about tuning learn the difference”
He knows the difference, and in fact likely knows way more then you about most truck and performance related things.
On a public forum, with a vast variety of readers, often things are best explained simply vs over-explained.

No need to jump in here and be a **** to him

 

I loved how the truck ran on the highway when I was running 315/75/16 tires. It's been a while, but I think the truck was turning 1800-1900ish rpm at 70mph. I'm now running 305/70/16 and turn about 2000-2100 at 70mph. Power loss was way more noticeable with the 315s though, but at the time, I was doing a lot of highway driving and I think I actually got slightly better mpg on the highway with the 315s compared to the 305s.

 

And there's the other disconnect about tire sizes....

315 vs 305 is only part of that equation

When reading tire sizes the first number,
315 or 305 is the width of the tread in millimeters.

A wider tire will make absolutely zero, ZERO, change to rotations at the ground per engine rpm in 100k miles. Heck throw a zero on the end, a tire 10 times as wide will still turn the same amount of times.

The number that makes the change is the second number, the aspect ratio. The 75 and 70 listed in your tire sizes above. That's the sidewall height listed as a ratio to the tire's width.

To make the math simple let's compare a theoretical 100/75/16 tire to a 100/70/16
So a 100millimeter wide tire with a 75 aspect ratio sidewall will be 75 millimeters from the 16" rim's bead to the tread
Next a 100 millimeter wide tire with a 70 aspect ratio sidewall will be 70 millimeters from the 16" rim's bead to the tread

Uh oh, you just lost 5 mm in tire height! Effectively changing your gear ratio, going from a 3.73 rear end to like a 3.87 ; it takes more engine rpm to cover the same amount of road distance. Great for towing where you want torque multiplication, bad for MPG where you want the max tire rotation possible from the same amount of spent fuel


Now that you can see how the middle tire number affects your "gear ratios" look at the real world tire size, 315 millimeters wide and 305 millimeters wide

A 315/75 will have a sidewall height of 236.25mm
A 305/70 will have a sidewall height of 213.5mm
You've changed the tire height by 22.75mm in that example. When you look at the resulting circumference changes, things get interesting.

A 236.25mm tire height is 742.2mm in circumference
A 213.5mm tire height is 670.7mm in circumference
The circumference change is 71.5mm, you've changed the amount of tire rotations necessary to travel the same distance by nearly 10%

​​​​​​So when you compare tire widths, you also need to look at the second number. A wide tire with a narrow sidewall can also be close to the same circumference as a narrower tire with a tall sidewall.
​​​​​​
Now a wider tire will have a wider contact pad, more rolling resistance and all that mess of math as well, I was just showing the difference in the overlooked tire height and how it pertains to it's effects on gear ratios, like the video in the post eluded to

 

You can't fault a guy for not knowing that, Wes. You wouldn't believe the amount of times I've been in tire shops where the SALESMAN is rattling off numbers to a customer and he has no idea what the numbers mean! Hell, look at craigslist tire ads - "255R18's for sale". 255 WHAT? There's no picture of the tire size but plenty of pictures of a bald tire that's 50% and a "good summer tire". The difference between 50/60/70, etc. really dictates if the tire will fit or not.

That being said, it's a shame there aren't any cheap 255/85-16s anymore. I remember buying the off-brand Cooper STs all the time. The last set I have here is on my plow truck from 2012; Trailcutter something or other.

 

This tire size chart is handy
https://tiresize.com/chart/

this rpm calculator is handy
https://www.crawlpedia.com/rpm_gear_calculator.htm

 

Ever since I've had my truck, I've had oversized tires on it, so I'm well aware of what the numbers mean...Did you read the whole post where I initially put 315/75/16 and 305/70/16? I typed "315s" and "305s" to abbreviate the tire size so that I wouldn't have to type out 315/75/16 and 305/70/16 every time. I only abbreviated it to save time. Didn't think I needed to type out 315/75/16 and 305/70/16 EVERY SINGLE TIME. But I guess next time I'll type up an excel sheet that shows what each number stands for in a metric tire size, as well as what size the tires are in freedom units. Then I'll also include what each number on a tire measured in freedom units stands for so that nobody bangs their head against a brick wall. Sound good?

 

Didn't mean to criticize you in particular, just posting it on a tire size thread.

Didn't intend to ruffle anyone's feathers, sorry all

 

I need to rotate these soon.

 

I run 315/75/16s with 3.73 gears and it tows great, it's all about having the right turbo, injector and tuning setup to make power in the RPM range you run in.

 

Similar thing to when you operate a vehicle on the dyno in the incorrect gear choice.
1:1 direct drive gives the most accurate results. Run the test in 3rd gear and you get “better” numbers.

 

3rd gear in an auto is 1:1 drive.

 

I see that you're running 180/80 injectors, what turbo are you running with that? Cause when I was running 315/75/16s, it put the rpms around 1800 at 70mph, and the stock turbo wasn't doing a whole lot of motivating at that rpm.